Logic control signals generated by the power-up reset signals of various power supplies are commonly used to determine the power-up sequence logic in an integrated circuit device. However, during power-up of an integrated circuit device that has multi-power domains, there is no sequencing control by the integrated circuit device over the external power supplies, and the traditional methods cannot control the sequencing of power supplies within functional blocks where specific power-up sequences are required.
The transitional instability of the power level of different power supplies during the power-up process is an important issue during the power-up process. Conventional level shifters can be used to transfer logic signals among various power levels when power supplies are stable; however, during power-up mode, not all power supplies are stable. In most cases, during power-up process, some power supplies become stable while others either continue to ramp up or remain inactivated. Specially designed level shifting circuits are needed to handle the power-up processes.